Production of vacua



Patented June 9, 1931 UNITED STATES. PATENT FFICE HERBERT J". WEST, OF GRAFTON, PENNSYLVANIA, ASSIGNOR TO THE SELIDEN COMPANY,

OF PITTSBURGH, PENNSYLVANIA, .A CORIPORATION OF.DELAWAR-E rRomIcrIoNor 'vAc'ua No Drawing.

This invention relates to methods of producing vacua, and particularly by means of high-vacua condensation pumps.

In the past high-vacua had been produced in condensation pumps using mercury as a circulating medium because of its moderately low vapor pressure and its stability. In the ordinary type of vacuum pump, where the vacuum desired is not as high, oils have ordinarily been used as a sealing medium. Such sealing medium does not require as high a resistance to heat as does the medium used in a condensation pump because the sealing medium in an ordinary type of low vacuum pump is not distilled and condensed, It has been proposed to use certain high oils in condensation pumps, and while these products possess in many cases a vapor pressure even lower than that of mercury, they have not proven successful as they tend gradually to break up on heating and are usually mixtures and do not have a constant vapor pressure at different rates of distilling so that-fluctuations in the distilling will result in vapors of different compositions and correspondingly condensates of different vapor pressures. Despite its high price and great weight therefore mercury has hitherto been practically the only medium used in condensation vacuum pumps.

According to the present invention esters of keto aromatic acids are used as a circulating medium for high-vacua condensation pumps or as a sealing and lubricating medium for the ordinary types of lower vacuum pumps. The esters of keto aromatic acids are either liquids or low melting solids which are extremely stable and possess very low vapor pressures. At the same time they are much lighter than mercury and volume for volume cost but an insignificant fraction of the cost of mercury. The lighter weight simplifies the construction ot'the pump and the much lower cost greatly decreases the investment required in the apparatus. I

. While the invention is not limited to any particular esters of keto aromatic acids, those of benzoylbenzoic acid and naphthoylbenzoic acid by reason of their relatively low price and excellent characteristics are pre- Application filed December 27, 1930. Serial No. 505,205.

ferred. However esters of homologues of esters as they are liquids at ordinary temperatures and have even lower vapor pressures. However the ethyl and methyl benzoylbenzoates melt at very low temperatures and if the apparatus is operated slightly above room temperature are very satisfactory.

Examples of the extremely low vapor pressures of the esters of the present invention may be noted that at a temperature of 161 C. the vapor pressures of ethyl benzoylbenzoate is only 1.5 m. m. of mercury, whereas the butyl benzoylbenzoate at 180 C. has a Vapor pressure of only 1.75 m. m. Esters of higher molecular weight have even lower vapor pressures, but since the vapor pressure of butyl benzoylbenzoate and butyl naphthoylbenzoate are suificiently low for satisfactory operation, it is normally unnecessary to use higher molecular esters which are considerably more expensive.

While the esters of the keto aromatic acids are particularly important in high-vacua pumps of the condensation type they of course may also be used as a sealing medium are directly applicable in the ordinary de signs of pumps. Naturally of course in condensation pumps the dimensions of orifices and the heating means must be suitably adjusted to the physical characteristics of the esters but no changein design is necessary.

While in the preferred embodiments of the present invention a single ester is used so that the uniform vapor pressure is achieved, it is possible to mi the esters oi the keto aromatic acids with other sealing media such as esters of phthalic acid and the like, and in its broader aspects the present invention includes the use of any sealing media which contain at least one ester of the keto aromatic acid whether or not this ester is the only component of the medium.

What is. claimed as new is:

1. In a method of producing high-vacua by means of condensation pumps the im provement which comprises using a pumping substance for providing the propellent vapor, one containing at least one ester of a keto aromatic acid.

2. A method according to claim 1, in which the ester is butyl benzoylbenzoate.

8. A method according to claim 1, in which the ester is a butyl naphthoylbenzoate.

4. In combination a pumping chamber, a supply of liquid containing at least one ester 5 of a keto aromatic acid, and means for vaporizing said liquid to establish a ct traversing said pumping chamber.

5. An apparatus according to claim 4, in which the ester of the keto aromatic acid is a butyl ester. 4

6. An apparatus according to claim 4, in which the ester is a butyl benzoylbenzoate.

7. An apparatus according to claim 4, in which the ester is a butyl naphthoylbenzoate.

Signed at Pittsburgh, Pennsylvania, this 26th day of December, 1930.

HERBERT J. WEST 

